The subject invention originates from twenty-two years research by the inventor, into engines and resonators that operate on the principles of thermoacoustic physics. For purposes of this application for patent, the term “thermoacoustic” refers to traveling energy impulses, normally detected as pressure fluctuations, propagating along velocity vectors that move thermal energy through an elastic medium that is typically a compressible working fluid. For purposes of this application for patent, thermoacoustic energy includes both shockwaves (supersonic and hypersonic pressure waves) and sound waves (pressure waves traveling at the sonic velocity of the working fluid under locally extant conditions).
The research background data in heat, acoustic wave phenomena and gas mechanics includes the shock tube research performed by government and institutional scientists during the 1950's and 1960's, relevant examples of which can be found in the Proceedings of the Seventh International Shock Tube Symposium, University of Toronto Press 1970, ISBN 0-8020-1729-0; as well as research into thermoacoustic waves generated by chemical explosives, The Chemistry of Powder and Explosives, Volume I, 1941, Volume II, 1943, by Tenney L. Davis, Ph.D., ISBN 0913022-00-4; published research in atmospheric physics, including Lightning, by Martin A. Uman, McGraw-Hill 1969; The Flight of Thunderbolts, 2nd ed., B. F. J. Schonland, Clarendon Press 1964; Graphic Survey of Physics, by Alexander Taffel, Oxford Book Company 1960; Matter and Motion, by James Clerk Maxwell, 1877, Dover Publications 1991 (reprint); Laboratory Exercises in Physics, Fuller and Brownlee, Allyn and Bacon 1913; Laboratory Experiments in Elementary Physics, by Newton Henry Black, Macmillan Company, 1944; Modern Physics, by Williams, Metcalfe, Trinklein and Lefler, 1968, Holt, Rinehart and Winston Publishers; Physics of Lightning, D. J. Malan, The English Universities Press Ltd., 1963; which includes thermoacoustic phenomena generated by natural lightning and man-made electric arcs.
Other relevant published research includes work in pulse tube refrigeration, including The Influence of Heat Conduction on Acoustic Streaming, Nikolaus Rott, Journal of Applied Mathematics and Physics (ZAMP), vol. 25, pp. 417-421, 1974; A Review of Pulse Tube Refrigeration, Ray Radebaugh, Cryogenic Engineering Conference, pp. 1-14, 1989; Flow Patterns Intrinsic to the Pulse Tube Refrigerator, J. M. Lee, P. Kittel, K. D. Timmerhaus, R. Radebaugh, National Institute of Standards and Technology, pp. 125-139, 1993.
The cryogenics department at NASA-Ames is a premier focus of pulse tube refrigeration research. Pulse tubes differ from most thermoacoustic devices in that they are typically non-resonant devices in which a mechanical piston, driven by an external power source, generates compression waves (pulses) that move in one direction through a series of heat exchangers, and cause thermal energy to be transported between those heat exchangers. Pulse tubes are typically used in cryogenic refrigeration applications. Pulse tubes are similar to most thermoacoustic devices in that traveling pressure waves in an elastic working fluid are the mode of operation.
The research history involving prime movers with associated thermoacoustic characteristics includes Stirling Cycle Machines, by Graham Walker, PhD, 1973, Oxford University Press; various Stirling engine technical research reports, 1937-1978, issued by The Philips Company Laboratories, Eindhoven, Netherlands; and Stirling Cycle Engines, by Andy Ross, 1977, published by Solar Engines, Phoenix, Ariz.
The device described herein is a traveling-wave Thermoacoustic Cycle (TAC) engine-generator set, comprised of a gas tight housing containing a compressible working fluid under pressure in which acoustic traveling waves are caused to propagate; a multiplicity of heat exchangers in which said acoustic traveling waves are amplified by a thermal gradient that causes said acoustic traveling waves to increase in pressure and temperature amplitudes and in wave propagation velocity; and an electrodynamic armature that said acoustic traveling waves impinge upon and cause to reciprocate within a magnetic field generating means in order to generate electrical energy.
TAC engines are well known to acoustic science, are in USPTO Class 310 and International Class H01L 041/08, and have been explored extensively by Peter H. Ceperley, George Mason University; Steven Garrett of Penn State University and Gregory Swift of Los Alamos National Laboratory. Thermoacoustic related patents searched include:
6,385,972May 2002Fellows 60/5176,054,775April 2000Vocaturo290/1R6,032,464March 2000Swift, et al 60/5175,953,920September 1999Swift, et al 60/520 X5,892,293April 1999Lucas 90/1R5,673,561October 1997Moss 62/65,659,173August 1997Putterman, et al250/3615,647,216July 1997Garrett 62/65,519,999May 1996Harpole, et al 60/520 X5,515,684May 1996Lucas, et al 62/65,456,082October 1995Keolian, et al 62/65,319,938June 1994Lucas 62/65,303,555April 1994Chrysler, et al 62/65,295,355March 1994Zhou, et al 62/65,275,002January 1994Inoue, et al 62/65,269,147December 1993Ishizaki, et al 62/4675,263,341November 1993Lucas 62/65,165,243November 1992Bennett 62/64,722,201February 1988Hoffler, et al 62/4674,686,407August 1987Ceperley 60/7214,599,551July 1986Wheatley, et al322/2R4,398,398August 1983Wheatley, et al 62/4674,355,517October 1982Ceperley 60/7214,114,380September 1978Ceperley 60/721
Standing wave Thermoacoustic Cycle (TAC) engines are typically comprised of a gas tight resonant cavity in the approximate shape of a cylinder, tube or torus, and internal isothermal heat exchangers that are separated by a regenerative heat exchanger (stack) and spaced along the length of the resonant cavity by a nominal wavelength or fraction thereof. An applied difference in thermal potential, across the length of the cavity, is created by a thermal gradient between the two isothermal heat exchangers. In standing wave resonators, the thermal gradient alone is sufficient to produce and amplify acoustic waves which transport thermal energy from one heat exchanger to another, and to maintain a state of oscillation, or periodic thermal and acoustic flux, in the working fluid.
To extract useful work from the engine, the oscillating pressure component can be applied to a mechanical member, such as a piston, in order to perform reciprocating work, and thereby used to perform tasks such as pumping fluids or generating electrical energy.
TAC engines have been researched for several decades, and researchers at the Los Alamos National Laboratory, the Naval Post Graduate School, The University of Texas, Penn State University and other institutions have written numerous research papers on the genre, primarily concerning standing-wave thermoacoustic physics. A standing-wave thermoacoustic refrigerator developed by Steven Lurie Garrett was flown aboard the space shuttle Discovery in 1991 as an experimental package. It is mentioned (project 511) along with this inventor's Acoustic Cycle engine (project 503) in the 1993 Rolex Awards For Enterprise, published December, 1992. Currently, there are approximately thirty relevant patents in the field.
The most significant problem with prior art thermoacoustic engines and refrigerators is that they have a very low power density. They are typically much larger and more massive for the amount of output work they produce, than other types of engines and refrigerators. Until 1998, disregarding non-resonant pulse tubes, most researchers working in the field, including Gregory Swift's Los Alamos group, concentrated their efforts largely on thermoacoustic engines that employed standing wave physics. The power output of standing wave systems is limited by the inherent physical characteristics, to wit; standing wave systems rely on the forward-going wave being inverted and reflected uniformly back along the resonator at nearly the same propagation velocity. If too much energy is extracted from the forward-going wave in the cold-side heat exchanger, the propagation velocity of the return wave is changed and the forward-going wave and the return wave will be out of phase and will interfere with each other. This adds impedance to the cycle and tends to damp the oscillation. This inherent characteristic severely limits the quantity of energy per cycle that is available to perform useful work, resulting in large engines with low power density.
Traveling-wave engines and pulse tubes, by comparison, do not rely on reflected waves to maintain system oscillation. Traveling-wave engines ideally eliminate the reflected wave, and propagate thermoacoustic energy in only one direction, thereby reducing the impeding effects of a change in wave propagation velocity on the system, and increasing the amount of useful energy that can be extracted from the system. All traveling-wave thermoacoustic engines, to some degree, experience a phenomenon known to practitioners of the art as “streaming”. The term refers to the physical motion of the working fluid. Where the desirable state of operation in most thermoacoustic engines is such that the mass of the working fluid resides in a static state, with the energy transport confined to pressure impulses (acoustic waves) that transit the working fluid, streaming is considered as an impedance; an undesirable effect. Streaming can be caused by convection currents and by physical displacement of the working fluid mass by the pressure impulses. This is typically problematic only in traveling-wave engines, where the energy impulses travel in only one direction.
In 1998-99, Greg Swift of Los Alamos attempted to improve on prior art by coupling Ceperley's torus-shaped traveling-wave engine with a cylindrical standing-wave resonator, in an effort to produce greater output power from the traveling-wave component, without damping the standing-wave oscillator. He also added mechanical elements to inhibit streaming. Even so, the compound engine develops low energy density because the design still relies mainly on conventional acoustics theory and geometry to produce an engine that is acoustically resonant.